The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material or to etch a layer of material in the fabrication of semiconductor devices.
Plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent the target impact the surface of the target to dislodge, i.e., xe2x80x9csputterxe2x80x9d material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths, from the target to the substrate, being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched openings, including trenches and holes of semiconductor devices having openings with a high depth to width aspect ratio, may not adequately coat the walls of the openings, particularly the bottom walls. If a large amount of material is being deposited, the deposited material can bridge over, causing undesirable cavities in the deposition layer. To prevent such cavities, sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively biasing (or self-biasing) the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term xe2x80x9cdense plasmaxe2x80x9d is intended to refer to one that has a high electron and ion density, in the range of 1011-1013 ions/cm3 
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in a steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance-matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
Although ionizing the deposition material facilitates deposition of material into high aspect ratio channels and vias, many sputtered contact metals have a tendency to deposit more thickly in the center of the wafer as compared to the edges. This xe2x80x9ccenter thickxe2x80x9d deposition profile is undesirable in many applications where a uniform deposition thickness is needed.
As described in application Ser. No. 08/680,335, entitled xe2x80x9cCoils for Generating a Plasma and for Sputtering,xe2x80x9d filed Jul. 10, 1996 and assigned to the assignee of the present application, which application is incorporated herein by reference in its entirety, it has been recognized that the coil itself may provide a source of sputtered material to supplement the deposition material sputtered from the primary target of the chamber. Application of an RF signal to the coil can cause the coil to develop a negative bias which will attract positive ions which can impact the coil causing material to be sputtered from the coil. Because the material sputtered from the coil tends to deposit more thickly at the periphery of the wafer, the center thick tendency for material sputtered from the primary target can be compensated by the edge thick tendency for material sputtered from the coil. As a result, uniformity can be improved.
It has been recognized that the sputtering rate for material sputtered from the coil may be nonuniform around the perimeter of the coil. Hence the ability to achieve a desired level of uniformity may be adversely affected in some applications.
It has further been recognized that the coil can develop a hot spot which can cause uneven heating of the substrate. This uneven heating of the coil can also cause reliability problems in that portions of the coil may become too hot and deform, and may also cause particulates deposited on the coil to flake off and contaminate the substrate. Since single turn coils are typically required to carry a relatively high level of current, these problems can be more pronounced in such single turn coils.
As set forth in copending application Ser. No. 08/857,921, entitled xe2x80x9cUse of Variable Impedance to Control Coil Sputter Distribution,xe2x80x9d filed May 16, 1997 and copending application Ser. No. 08/857,720, entitled xe2x80x9cUse of Variable Impedance to Control Coil Sputter Distribution,xe2x80x9d filed May 16, 1997 and assigned to the assignee of the present application, which applications are incorporated herein by reference in their entirety, this xe2x80x9chot spotxe2x80x9d on the coil may be shifted around the coil as the deposition progresses by repetitively changing one or more impedances coupled to the coil. As a result, the coil may be more uniformly heated and sputtered. However, changing the inductance of an inductor (or the capacitance of a capacitor) coupled to the coil may not be appropriate in some applications. Accordingly, a need exists for additional processes and apparatus which can improve the uniformity of deposition of ionized material onto a substrate.
These and other objects and advantages are achieved by a plasma generating apparatus in which, in accordance with one aspect of the invention, the output of an RF generator coupled to an RF coil is varied to shift RF voltage distributions along the length of the RF coil. It has been noted that the RF voltage distributions along the coil influence the plasma properties such as the plasma density and potential profiles and electron and ion movements including ion bombardment of the coil and substrate being deposited. It has further been noted that the instantaneous RF voltage distributions along the coil are not uniform or symmetric about the symmetry axis of the coil. These nonuniform and asymmetrical instantaneous RF voltage distributions along the coil can lead to nonuniform and asymmetrical heating of both the coil and the substrate as well as nonuniform sputtering of the coil and nonuniform deposition of material on the substrate.
In accordance with one aspect of the present invention, it is recognized that varying a parameter of the RF signal applied to the coil during a sputtering operation can move or vary the RF voltage distributions along the RF coil so that minima and maxima points of the RF voltage distribution along the coil are not fixed at particular regions of the coil. Instead, the RF voltage distribution can be repeatedly moved around the coil in a rotational or other motion. In addition, the ionization pattern of the plasma associated with the RF voltage distribution may be similarly moved in conjunction with the movement of the RF voltage distribution. As a consequence, the RF coil and substrate can be more uniformly and symmetrically heated, by time-averaging, because a xe2x80x9chot spotxe2x80x9d of sputtering can be avoided. In addition, the coil itself may be more uniformly sputtered and the deposition material can be more uniformly deposited.
In one embodiment, the RF generator is programmed so that the frequency of the RF signal cyclicly shifts to various frequencies. In an alternative embodiment the power level of the output is cyclicly shifted. In this manner, the voltage distributions along the coil may be cyclicly shifted to improve uniformity.